Several lines of evidence indicate that neuronal connections in the mammalian nervous system are capable of substantial change. This evidence includes the normal rearrangement of synaptic connections during development, the continued capacity for synaptic rearrangement in response to experimental manipulation, and ultrastructural signs of physiological remodelling throughout life. The purpose of the experiments proposed here is to develop and apply several methods that will enable us to monitor the geometry of individual neurons for periods of up to a week or more in mammalian autonomic ganglia. Our more general aim is to obtain direct evidence about the way in which connections between mammalian nerve cells are regulated. The methods we will use include intracellular marking by a) concurrent injection of the fluorescent dye 6-carboxyfluorescein and horseradish peroxidase; b) injection of dextran labeled with fluorescein; and, c) supravital staining of nerve cells using intracellular peroxidase to catalyze the production of melanin from DOPA. The first two methods involve imaging neurons in vivo by SIT camera video intensification. The last approach may allow living neurons containing melanin to be observed repeatedly with conventional optics. Autonomic ganglia will be used for these studies because of their accessibility, and because we already know a great deal about the connections and development of these relatively simple collections of neurons. We believe, however, that the techniques and principles which may emerge from this work will be useful in understanding other more complex parts of the mammalian nervous system.